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FIT calculations

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Robin Clark 2010-12-16 09:16:20 +00:00
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fmmd_design_aide/fmmd_design_aide.tex
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@ -494,9 +494,69 @@ We can surmise the symptoms in a list.
\item symptom: \textbf{no~test~effect} caused by the failure modes: no~test~effect, low~reading.
\end{itemize}
\section{MTTF Reliability statistics}
\section{conclusions}
%\clearpage
\subsection{OP-AMP FIT Calculations}
The DOD electronic reliability of components
document MIL-HDBK-217F\cite{mil1992}[5.1] gives formulae for calculating
the
%$\frac{failures}{{10}^6}$
${failures}/{{10}^6}$ % looks better
hours for a wide range of generic components.
These figures are based on components from the 1980's and MIL-HDBK-217F
gives very conservative reliability figures when applied to
modern components. The formula for a generic packaged micro~circuit
is reproduced in equation \ref{microcircuitfit}.
The meanings of and values assigned to its co-efficients are described in table \ref{tab:opamp}.
\begin{equation}
{\lambda}_p = (C_1{\pi}_T+C_2{\pi}_E){\pi}_Q{\pi}_L
\label{microcircuitfit}
\end{equation}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Low SIL assessment MAX3053
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{table}[ht]
\caption{OP AMP FIT assessment} % title of Table
\centering % used for centering table
\begin{tabular}{||c|c|l||}
\hline \hline
\em{Parameter} & \em{Value} & \em{Comments} \\
& & \\ \hline \hline
$C_1$ & 0.040 & $300 \ge 1000$ 1001 BiCMOS transistors (1214) \\ \hline
${\pi}_T$ & 1.4 & max temp of $60^o$ C\\ \hline
$C_2$ & 0.0026 & number of functional pins(8) \\ \hline
${\pi}_E$ & 2.0 & ground fixed environment (not benign)\\ \hline
${\pi}_Q$ & 2.0 & Non-Mil spec component\\ \hline
${\pi}_L$ & 1.0 & More than 2 years in production\\ \hline
\hline \hline
\end{tabular}
\label{tab:opamp}
\end{table}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% END OF Low SIL assessment MAX3053
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Taking these parameters and applying equation \ref{microcircuitfit},
$$ 0.04 \times1.4 \times0.0026 \times2.0 \times2.0 \times1.0 = .0005824 $$
we get a value of $0.0005824 \times {10}^6$ failures per hour.
This is a worst case FIT\footnote{where FIT (Failure in Time) is defined as
failures per Billion (${10}^9$) hours of operation} of 1.
\subsection{Switching Transistor}
The switching transistor will be operating at a low frequency
and well within 50\% of it maximum voltage.
MIL-HDBK-217F\cite{mil1992}[6-25] gives an exmaple
transistor in these environmental conditions, and assigns an FIT value of 11.
\section{Conclusions}
With the safety addition the undetectable failure mode of \textbf{low~reading}
disappears.